Coated pyrotechnic metal powders and method of their preparation



United States Patent COATED PYROTECHNIC METAL POWDERS'AND METHOD OF THEIR PREPARATION Raymond H. Comyn, Silver Spring, Md.

No Drawing. Application July 1, 1952 Serial No. 296,769

3 Claims. ci. 148-6.2)

(Granted under Title 35, U. 8. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This application is a continuation-in-part of application Serial No. 213,661, filed March 2, 1951, now abandoned.

This invention relates to protective coatings for metal powders for protecting the powders against deterioration and more particularly to a process for applying a'thin coating of water and chemical impervious substance to the surfaces of such powders.

Methods heretofore employed for protecting metal powders against deterioration have not proved to be entirely satisfactory for the reasons that such coating only protected the powders against deterioration for a shortor limited period of time and the thickness of the coating could not be closely controlled. Furthermore; such a coating would flake off and thus expose the metal surfaces to oxidation. j

Accordingly, it is an object of the present invention to provide a new and improved protective'co'ating of controlled thickness for metal powders which will prevent deterioration of the powder.

Another object of the invention is to provide a coating for metal powders which will prevent other chemicals lying in close proximity thereto from reacting with the metal powders.

Still another object of the invention is to provide a method of producing a thin coating of water and chemical impervious substance to the surface of each particle of the metal powders for preventing deterioration and reaction thereof with other chemicals.

A further object of the invention is to provide a new and improved method of producing a thin coating of.

water repellent material on metallic powders which will not affect the subsequent use of the powder and which is reproducible so that the properties of the powders may be reproducible.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description.

It is well known that metal powders such, for example, as cobalt, manganese and the like are employed extensively in compounding pyrotechnic mixtures, particularly gasless delay mixtures for use .in ordnance devices such, for example, as shells, rockets and the like. i

Such metals as heretofore mentioned are commonly known in the trade as metal fuels and are used in pyrotechnic mixtures for causing proper delay and uniform burning time of the mixture when the mixture has been ignited. Furthermore, it is well known that the aforesaid fuels are readily affected by atmospheric conditions such, for example, as high humidity and temperature and thus causing erratic burning of the pyrotechnic in which such fuels are used.

The purpose of the present invention is to improve Patented Apr. 29, 1958 the moisture-resistance characteristic of metal powders by coating the powders with a thin layer of water-repeb lent chemical of controlled thickness whereupon the properties of the powders are reproducible and thus subsequent use thereof is not affected, the powder also being protected against deterioration thereby increasing the life thereof and making the powders admirably suitedv for use in pyrotechnic-mixtures are employed extensively in ordnance devices. Furthermore, it has been found that ordnance devices employing mixtures having the new and improved metal powders therein are extremely stable and thus can be stored over a long period of time without appreciable deterioration of the mixture due to adverse' atmospheric conditions.

It is well known in the art of metal powders that moisture accelerates the deterioration thereof, such deterioration resulting from atmospheric conditions such, for example, as high humidity and temperature. In view of these unfavorable conditions existing in prior art of the metal powder in a warm aqueous solution of a soluble dichromate as for example, sodium or potassium dichromate. It has beendetermined that a 5% solution. of the dichromate at a temperature between 70 and C.

may be used to treat th'e'metal powder. The metal pow der is permitted to remain in thedichromate solution for a period of time approximately thirty minutes and is thereafter filtered in any suitable manner, washed with distilled water and dried. The result of this treatment-is to leave a film on the powder which provides protection against atmospheric corrosion and conditions the powder for .the application thereto of stearic acid. The above; treatment results in an increase in the weight of the. powdered metal. In one particular case, a weight increase of about 5% was noted after manganese powder was treated with a soluble dichromate and then dried, thus.

suggesting that the metal powder has reacted with the dichromate solution, probably forming an oxide coating. on the powder.

The dried dichromate treated powder is then treated with stearic acid by stirring the dry powder in a solution of stearic acid and a suitable solvent such, for example, as

ethyl ether or carbon tetrachloride for a predetermined period of time. Any desired concentration may be used.

and the time is not critical. When this operation has been completed the powder is again filtered and washed stantially a molecule thick over the film resuiting from the dichromate treatment thereby to protect the powder against oxidation whereupon deterioration of thev pow der due to adverse atmospheric conditions is greatly re duced.

It has been found by actual tests that neither a di 'chromate treatment nor a stearic acid treatment alone affords the desired protection for the powder against de-v terioration while on the other hand it has been found that the improved method of treatment as heretofore set forth produces the desired result. For example, during these tests a powder such, for" example, as manganese,-

was employed and treated in the above mentioned manner and in each instance the treated powder was exposed to relatively high humidity of 95% and temperature of 71 C. A test on untreated powder under the afore said conditions was also made. However, during such tests the treated and untreated powders were dried at predetermined intervals andweighed. Thereafter the relativedeterioration was measured by percent weight increase.

The results of these tests are shown onthe following chart which clearly discloses that the improved method of treating metal powders as set forth herein provides a protective coating for metal powders which is far superior in the essential characteristics of preventing oxidation'thereof, than those disclosed by prior art methods.

Rerce n r weight increase of dried powder after exposure at 71 C., 95 relative humidity for indicated period Treatment 12 Days 24 Days Exposure Exposure Percent Perce N one 3. 25 5. 24 Dlchromate only 88 1. 61 Stearic Acid only 1. 21 3. 75 Dlchromate followed by Stearic Acid 10 29 After a careful study of the foregoing chart it will be apparent that the untreated metal powderdete'riorated a considerable amount during the exposuretime of 12 and 24 days and that the powder treated with either dichrornate or. stearic acid 'alone' also showed, considerable dterior ation due to oxidation over such period of time. I Whil'e'onthejother hand, the powder-treated in the manneras heretofore'set forth, first with dichromate followed by'stearic" acid tr'eatment, -deteriorated comparatively litl tl'e over'the same period oftimejas comparedto the un': treated powder," dichromate, Qrstearic acid treatedpowmoleculesare adsorbediby the film on the metal resultmhti'eftreated metal powder are covered: with the stearic'r acid molecules, the attraction. of additional molecules thereto ceasesan'd thus after the powder has been'stirred inf'astearic acid solution for'ja sufiicientperiod of time; the surface thereof is covered therewith whereupon no',' further stearic acid will adhere thereto. Furthermore, if I I I I I t u' m repcatemwashing's of thief b m 50 M =284.5=gm'.' gram-molecular weight of stearlc acld honed treated powder such',"f0r examplejasf app'rox1- I M molecule I mately 200': washings. with fresh solventg'lphave" failed to remove the protective coating from thei'powder. How

ever, a lesser number of wash treatments areljsufiicientgs to remove the excess stearic acid. For example; by 10' washings with fresh solvent, the excess stearic' acid is sub.

stantially' removed from the powder leaving the strongly adsorbed stearic acid on the dichromate treatedmetal powder thereby providing a and uniform protective coating of controlled thickness on the particle surfaces der. As an example of the process of this invention; pow-f continued" for approximately 30 minutes. The metal, powder is then removed from the solution by filtering.-

and thereafter washed with distillediwaterand dried.j

' Wsta,0,0Q25X-=329X10-' grams of etearic acid.

The-dry treatedpowder resulting from the preceding step'is then stirred in a solution of stearic' acid in carbon tetrachloride.- Thereafter' the powder is again filtered'andwashed with" carbon tetrachloride tie-remove the exeessstearicacid If the intended use ofithe powder is such that-the 'excess'stearic acidis notobjectionable' the bonanalysesinihe ,table indicate equal amounts of care0 of'thecoate'd thereof; which will not affect subsequent useof the pow '1 cobalt as well as with manganese and process may also I be carried out by substituting sodium dichromatc for the potassium dichromate of. the above example.

The reproducibility of the coating will be clearly apparent after a study of the following chart on which is shown the results of tests of two samples of manganese treated first with a dichromate solution and then with a stearic acid solution, the so-treatcd samples thereafter being washed thoroughly with a fresh solvent solution the results of which are also shown on the chart. It

' will be noted that the carbon content of the powder increased considerably after the dichromate and stearic acid treatment.

Percent Calculated I Percent Carbon After Percent Manganese Lots Carbon After Subsequent Carbon Dichromate Stearic Acid Added by Treatment Treatment Stcarlc Acid Treatment The percentage of carbonladded by the stearic acid treatment providesa measure of the steanc acid adembed on the surface of the metal powder.

30 .Since the quantityof stearic acid added to the surface ofi a'r'netalpoyvderby the aforedescribed treatment probably de'pendsonthe surface areaof the powder, it wouldf l'befexpected' 'Ithat a uniform coating procedure wouldiresultiin'ariequal addition of stearic acid to each of two lots of manganese. The results of the carbon, ditherieforeilequaliamounts ofstearic acid are added sampleszof the two lots of manganese. The maximumstearic acid that be added to either lotis given. by the following 6qllatl0Il ,:as5l1ining that the stearic acid is present a monomolecularfilm:

' '45 W=gmljjtiearie acid adsorbediincom lete monolayer ,;Av ogadro Number 6.06X10 niolecules/gm. mol. -=-.-:19s5= 2s4.5 s: j I Y 10 a gram stearlc 801d absorbed in complete monolayer on one gram of Lot 2007! on 2008manganese I I The 'steariir aicid actually adsorbed in one gram of man gancse powder can becalculated'from the carbon analyses powder by the following formula:

Wst=grams etearic ricidadsorbed in one gram manganese 05 W,- =.00034 carbonper gram of manganese after adsorption-of stea'rie acid -W, .00010. gnu carbonper gram of manganese beforegwatdsorptionpf stearic acid ism' i l arbon in one mol. of stearic acid I 2 v 2165 I I I adsorbed in dire gram of'hot 2007 manganese 284.5 'g mt grammol'ecular weight of stearic acid it completely with a monomolecular layer, there is certainly not suflicient stearic acid to cover the manganese with more than a monomolecular layer. Actually, the experimental error may be large enough to account for the fact that there does not appear to be sufiicient stearic acid to cover the manganese completely.

From the foregoing, it will be apparent that a new and improved protective surface coating for metal powders has been provided which is far superior in deterioration-resistance characteristics than any of the coatings heretofore employed. Furthermore, from the foregoing it will be understood that a thin and uniform water repellent material of controlled thickness is applied to the particles of the metal powder, such coating being sufliciently thin so as not to affect subsequent use of the powder, the coating also being reproducible so that the properties of the powder are reproducible and thus the powder is admirably suited for use in pyrotechnic mixtures. Whereas, a more stable fuel is provided for use in a pyrotechnic mixture which will not be readily affected by atmospheric conditions and thus assuring proper delay and uniform burning time of the mixture after being stored over a long period of time under adverse weather conditions.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A metal fuel for use in pyrotechnic mixtures and the like comprising a metal powder selected from the group consisting of cobalt and manganese, said metal powder having an inner coating of the oxide of said metal formed by the reaction of said metal with a warm aqueous solution of a dichromate selected from the group consisting of sodium dichromate and potassium dichro mate, and said metal powder having an outer coating formed by the reaction of said inner coating with a solution of stearic acid.

2. A metal fuel for use in pyrotechnic mixtures and the like comprising a finely divided metal selected from the group consisting of cobalt and manganese, said finely divided metal having an inner coating formed by the reaction of the metal with a warm aqueous solution of a dichromate selected from the group consisting of sodium dichromate and potassium dichromate, and said finely divided metal having an outer coating formed by the reaction of said inner coating with stearic acid.

3. The method of providing pyrotechnic metal powders, such as cobalt, manganese and the like, with a protective water repellent coating, comprising immersing the metal powder in an aqueous solution containing about 5% of a dichromate selected from the group consisting of sodium dichromate and potassium dichromate at a temperature of from about C. to about C. for a period of about 30 minutes, drying said powder, and subsequently immersing said powder in a solution of stearic acid in an organic solvent.

References Cited in the file of this patent UNITED STATES PATENTS 639,867 Rosenberg Dec. 26, 1899 1,869,041 Bengston July 26, 1932 2,206,064 Thompson et al. July 2, 1940 2,279,252 Slunder Apr. 7, 1942 2,454,799 Hart et al Nov. 30, 1948 FOREIGN PATENTS 218,619 Great Britain 1925 518,966 Great Britain Mar. 13, 19.40

579,165 Great Britain July 25, 1946 

1. A METAL FUEL FOR USE IN PYROTECHNIC MIXTURES AND THE LIKE COMPRISING A METAL POWDER SELECTED FROM THE GROUP CONSISTING OF COBLAT AND MAGANESE, SAID METAL POWDER HAVING AN INNER COATING OF THE OXIDE OF SAID METAL FORMED BY THE REACTION OF SAID METAL WITH A WARM AQUEOUS SOLUTION OF A DICHROMATE SELECTED FROM THE GROUP CONSISTING OF SODIUM DICHROMATE AND POTASSIUM DICHROMATE, AND SAID METAL POWDER HAVING AN OUTER COATING FORMED BY THE REACTION OF SAID INNER COATING WITH A SOLUTION OF STEARIC ACID. 